Most aircraft and helicopter engines use dual magneto systems to provide current impulses to the engine spark plugs during normal operation. In the event of a magneto failure, the remaining magneto in the pair continues to provide current impulses to maintain engine performance. Once a magneto is in operation, no external power is required to maintain the magneto's operation. Only during engine start-up is an external power supply, usually battery power, used to rotate the magnetos by turning the engine over by the engine starter motor. Each magneto has a so-called "P-lead" which, if grounded, disables the magneto from providing current impulses to the engine spark plugs.
Some aircraft ignition systems provide for boosted engine ignition during the engine start-up sequence through the use of a battery operated spark booster device, commonly known as a starter vibrator or "shower of sparks" for the first magneto only. The shower of sparks ignition system is described in the publication entitled "The ABC's of the TCM Shower of Sparks Ignition System" by Teledyne Continental Motors, the manufacturer. The starter vibrator or shower of sparks provides an enhanced AC current to the first magneto to supplement the single current impulses obtained from the first magneto to facilitate engine start-up. A retard breaker circuit is also activated in the first magneto during engine start-up which delays the function of the running points via the retard or starting points so that the starting points will not effectively open until very near top dead center, or "TDC". In the normal operation of the first magneto the running points, also known as the switch or advance points, are set to open 22.degree. before top dead center, or "BTDC". This relative delay in the opening of the starting points as compared to the running points prevents kick-back which occurs if the spark plugs spark prematurely during the relatively slow rotational speed of the magnetos during engine start-up. The second magneto is grounded during engine ignition to prevent the second magneto from providing current impulses via its running points. This prevents engine kick-back.
When the pilot is prepared to attempt engine ignition, the starter switch, a plurality of tandem switches operating simultaneously, is engaged whereby the second magneto is grounded through its P-lead, and the starter vibrator and the retard breaker circuit are activated for the first magneto. Simultaneously, a battery operated engine starter motor turns the engine over which also causes the first and second magnetos to rotate. Only the first magneto's starting/running points provide current impulses, to which the enhanced AC current from the starter vibrator is added, which is normally adequate to initiate engine ignition. Once the engine starts, the starter switch is disengaged which deactivates the engine starter motor and breaks the grounding circuit for the second magneto causing it to provide current impulses via its running points. The starter vibrator and the retard breaker circuit are also simultaneously deactivated once the starter switch is disengaged and the first magneto also then provides current impulses via its running points. The engine continues to operate until and unless the magnetos are grounded via their respective P-leads at normal engine shut-down.
In the event of a malfunction of the starter vibrator or the first magneto starter circuit, the engine will not start. The aircraft must then be serviced by a mechanic. This delays the aircraft and, if parts are unavailable to replace or repair the malfunction, the delay may be considerable. Small aircraft are more likely to fly to remote areas or smaller airports where mechanics and replacement parts are not readily available. Adverse conditions, such as inclement weather especially during the winter months, may also prevent the engine from starting normally. Since a battery is used to power both the engine starter motor and starter vibrator, a limited opportunity is available to initiate engine ignition before the battery is depleted.